For tires for passenger vehicles and tires for sport utility vehicles in which the contact pressure per unit area is low, the tread portion contacting the road surface needs to have, at the same time, good abrasion resistance, wet grip performance and fuel economy, and in particular good abrasion resistance.
As the method for improving abrasion resistance, techniques that involve improving wet grip performance, fuel economy and abrasion resistance using silica having a large nitrogen adsorption specific surface area (finely divided silica) or carbon black having a large nitrogen adsorption specific surface area (finely divided carbon black) are being introduced (for example, Patent Literature 1).
Unfortunately, finely divided silica and carbon black have a strong tendency to agglomerate and are thus difficult to uniformly disperse. Hence, silica manufacturers try to improve the dispersibility of silica in a rubber composition by adjusting the surface activity and particle distribution of silica. However, in the case of a rubber composition containing a modified polymer for example, at an initial stage of kneading, the modified polymer may be bonded to silica, inhibiting dispersion of silica. Thus, finely divided silica and carbon black are difficult to uniformly disperse, and addition of finely divided silica or carbon black rather tends to reduce elongation at break.
Another method for improving abrasion resistance is to reduce the sulfur dosage and use a sulfur-containing crosslinking agent such as 1,6-bis(N,N′-dibenzylthiocarbamoyldithio)hexane, bis(3-triethoxysilylpropyl)tetrasulfide, and poly-3,6-dioxaoctane-tetrasulfide. Unfortunately, these sulfur-containing crosslinking agents are generally expensive while they can only improve abrasion resistance by about 10%. Further improvement in abrasion resistance is thus demanded.
Still another method for improving abrasion resistance is to form silica (or carbon black) and a portion of a polymer component into a masterbatch. This method, however, allows improvement in abrasion resistance by about 10%, and further improvement in abrasion resistance is thus demanded.
Yet another method for improving abrasion resistance is to introduce a modifying group interactive with silica into the end or backbone of styrene butadiene rubber to improve the dispersibility of silica. This method, however, allows improvement in abrasion resistance by about 10%, and further improvement in abrasion resistance is thus demanded.
As the method for improving abrasion resistance, a technique is also known in general in which approximately 10 to 35% by mass of a high-cis butadiene rubber synthesized with an Nd catalyst is used based on 100% by mass of the rubber component to take advantage of good abrasion resistance of the butadiene rubber itself. Unfortunately, this technique may cause a great reduction in wet grip performance in tires for passenger vehicles and tires for sport utility vehicles in which the contact pressure is low. For this reason, the amount of the high-cis butadiene rubber used is thought to be limited to not more than about 35% by mass based on 100% by mass of the rubber component.
As mentioned above, it is desired to provide a rubber composition for a tread that has both excellent abrasion resistance and good wet grip performance and also offers good fuel economy and elongation at break.